Prior to the publication of Constructing Accessible Web Sites, I published an early version of my chapter
on testing tools. The first section of that chapter, Testing Tools,
outlines what can and what cannot be tested relative to the Section 508 Web Accessibility Standards. Although Gez Lemon doesn't phrase
it that way, what he has done is something I wanted to do; he has created a web page that exemplifies violations
of the WCAG checkpoints (instead of Section 508) that will probably not be detected as errors by the testing tools.
That will be a great resource for training in accessibility. We all know it is possible to fool testing tools, and
Gez Lemon's example page provides a wonderful set of examples and a wonderful
teaching tool.

Testing tools are important. The simple fact is that there is a high correlation between errors that can be checked
by software like missing alt text and form controls without labels and those that can't be checked. Perhaps more
important, missing alt text and form controls without labels are very important errors .Testing tools can catch these
errors much better than a human reviewer can and these errors are important. The process at IBM of monitoring
over 13 million pages with automated accessibility checking over the last several years has proved effective in improving
the overall accessibility of the IBM internet and intranet web presence - but the necessity of human review is without
question and will be discussed in a forthcoming article in the IBM Systems Journal, Managing usability for people with disabilities in a large Web presence.

My only criticism of the Gez Lemon article is the suggestion that his results represent the testing tools. In fact
they represent only the free single page accessibility checkers, which he called the "leading accessibility
validators."
You do get what you pay for, and the warnings offered by the real products might have pointed to the possibility
of errors in cases he presents.

Since posting this, Gez has contacted me to say that he has modified his article to say that "the test page
was tested with accessibility validators that are free to use."

The document that breaks the rules is http://juicystudio.com/experiments/invalid.html. It is a great
resource for trainers to emphasize the importance of human review. Stay tuned here for a study of what the real accessibility
validators do with what I consider to be checkable accessibility violations. Of course I consider that a much more
interesting question.